The invention relates to a circuit for demodulating two-dimensional data symbols transmitted by carrier-based data transmissions.
The invention applies to the field of demodulating two-dimensional data symbols and thus concerns virtually all modern methods relating to carrier-based data transmission or data transmission on the basis of a carrier frequency (DVB-T, DVB-S, DVB-C, DAB, ADSL, etc.). Nowadays, it is usual to carry out demodulation using digital signal processing.
It is accordingly an object of the invention to provide a circuit for demodulating two-dimensional data symbols transmitted by a carrier-based data transmission that overcomes the disadvantages of the prior art devices of this general type, which permits the sampling frequency of a analog/digital converter to be selectable independently of the symbol rate and which ensures that interfering spectral components in the output signal from the tuner are not convolved into the useful band as a result of sampling. Hence, simple measures in the digital quadrature demodulator are intended to ensure that DC components at the output of the analog/digital converter do not impair demodulation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for demodulating two-dimensional data symbols transmitted by a carrier-based data transmission. The circuit has a tuner with a first intermediate frequency stage for converting a carrier frequency of a reception-side input signal containing the two-dimensional data symbols to a first intermediate frequency f1. A surface wave filter is connected downstream of the first intermediate frequency stage. A second intermediate frequency stage for converting the carrier frequency of the input signal to a second intermediate frequency f2 being lower than the first intermediate frequency f1 is provided and is connected downstream of the surface wave filter. A quadrature demodulator is connected downstream of the tuner and converts the input signal at the second carrier intermediate frequency f2 to a baseband and splits it into in-phase and quadrature components. The quadrature demodulator has an A/D converter with a prescribed sampling frequency fa and an anti-alias filter is connected upstream of the A/D converter for removing interfering spectral components. The first intermediate frequency f1, the second intermediate frequencie f2 and the sampling frequency fa being approximately defined by one of the following equations: f2=f1/5 and fa=4xc2x7f2; f2=f1/5 and fa=4/5xc2x7f1; fa=4/5xc2x7f1; or fa=6/5xc2x7f1.
Accordingly, the invention is able to overcome the aforementioned problems elegantly, i.e. in a surprisingly simple manner, by suitably stipulating the intermediate and sampling frequencies.
In other words, the invention is based on recognition of a mathematical correlation indicating how the sampling frequency of the digital quadrature demodulator and the intermediate frequency/frequencies of the tuner needs/need to be selected in order to change the alias components. The alias components are changed by sampling the oscillator crosstalk which can arise in the second stage of the tuner, to a particular frequency independently of the symbol rate of the data signal. This particular frequency is intended to ensure that the interference signal can be suppressed in the digital quadrature demodulator using simple measures. The DC component at the output of the analog/digital converter is then automatically rendered harmless.
The latter effect is also effective for receiver configured using direct sampling of a single intermediate frequency of the tuner.
In accordance with an added feature of the invention, the anti-alias filter is a filter selected from the group consisting of low-pass filters and bandpass filters.
In accordance with an additional feature of the invention, the quadrature demodulator has a removal device for removing a frequency fa/4 and is connected downstream of the A/D converter.
In accordance with another feature of the invention, the removal device has a mixer low-pass filter configuration with a frequency trap at fa/4.
In accordance with another added feature of the invention, the removal device has a mixer low-pass filter configuration with a zero.
In accordance with a concomitant feature of the invention, the quadrature demodulator has a mixer low-pass filter configuration connected downstream of the A/D converter and a sampling rate converter with a low-pass filter connected downstream of the mixer low-pass filter configuration.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit for demodulating two-dimensional data symbols transmitted by a carrier-based data transmission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.